netdev - Re: Network performance with small packets

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Date:	Tue, 01 Feb 2011 23:03:46 -0800
From:	Shirley Ma <mashirle@...ibm.com>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	mashirle@...ux.vnet.ibm.com, "Michael S. Tsirkin" <mst@...hat.com>,
	netdev@...r.kernel.org, netdev-owner@...r.kernel.org,
	Sridhar Samudrala <sri@...ibm.com>,
	Steve Dobbelstein <steved@...ibm.com>
Subject: Re: Network performance with small packets

On Wed, 2011-02-02 at 12:04 +0530, Krishna Kumar2 wrote:
> > On Tue, 2011-02-01 at 22:05 -0800, Shirley Ma wrote:
> > >
> > > The way I am changing is only when netif queue has stopped, then
> we
> > > start to count num_free descriptors to send the signal to wake
> netif
> > > queue.
> >
> > I forgot to mention, the code change I am making is in guest kernel,
> in
> > xmit call back only wake up the queue when it's stopped && num_free
> >=
> > 1/2 *vq->num, I add a new API in virtio_ring.
> 
> FYI :)

> I have tried this before. There are a couple of issues:
> 
> 1. the free count will not reduce until you run free_old_xmit_skbs,
>    which will not run anymore since the tx queue is stopped.
> 2. You cannot call free_old_xmit_skbs directly as it races with a
>    queue that was just awakened (current cb was due to the delay
>    in disabling cb's).
> 
> You have to call free_old_xmit_skbs() under netif_queue_stopped()
> check to avoid the race.

Yes, that' what I did, when the netif queue stop, don't enable the
queue, just free_old_xmit_skbs(), if not enough freed, then enabling
callback until half of the ring size are freed, then wake the netif
queue. But somehow I didn't reach the performance compared to drop
packets, need to think about it more. :)

Thanks
Shirley

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